Problem: The scale on a map is 10cm : 1km. If two cities are 30cm apart on the map, what is the actual distance in km between the two cities?
The scale means that every 10cm on the map corresponds to an actual distance of 1km. A distance of 30cm on the map is the same as 3 $\cdot$ 10cm. The actual distance between the two cities is 3 $\cdot$ 1km, or 3km.